fix(viewer): replace text-accent with text-primary on annotation toggle inactive state

Fixes WCAG 2.1 AA contrast failure (#341): text-accent (#a1dcd8) on light
PDF control bar was 1.52:1 — well below the 4.5:1 AA minimum. text-primary
resolves to #012851 in light mode (14.5:1) and #a1dcd8 in dark mode (9:1) —
both states pass AA in both themes.

Adds PdfControls.svelte.spec.ts with 5 tests covering toggle visibility,
label strings, and the contrast-safe class assertion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/PdfControls.svelte

@@ -91,7 +91,7 @@ let {
 			aria-label={showAnnotations ? m.pdf_annotations_hide() : m.pdf_annotations_show()}
 			class="flex items-center gap-1.5 rounded px-2 py-1 font-sans text-xs transition {showAnnotations
 				? 'text-ink-2 hover:bg-surface/10'
-				: 'bg-surface/10 text-accent'}"
+				: 'bg-surface/10 text-primary'}"
 		>
 			<svg
 				class="h-3.5 w-3.5 shrink-0"